This invention relates to improvement in the production rate and improvement in the raw material and energy utilization of the carbothermic reduction of silicon dioxide to produce silicon. More specifically. this invention relates to the addition of calcium compounds to achieve these improvements. This invention also relates to the improved preparation of silicon carbide.
Silicon is typically produced via the carbothermic reduction of silicon dioxide (SiO.sub.2) with a solid carbonaceous reducing agent. The silicon dioxide may be in the form of quartz, fused or fume silica, or the like. The carbonaceous material may be in the form of coke. coal, woodchips. and other forms of carboncontaining materials. The overall reduction reaction is represented by the equation EQU SiO.sub.2 +2C=Si+2CO.
It is generally recognized that the above reaction in reality involves multiple reactions. the most significant being outlined below: EQU SiO.sub.2 +3C=SiC+2CO (1), EQU SiO.sub.2 +C=SiO+CO (2), EQU SiO+2C=SiC+CO (3), EQU 2SiO.sub.2 +SiC=3SiO+CO (4), EQU SiO+SiC=2Si+CO (5),
and EQU SiO.sub.2 +Si=2SiO (6).
Experimental work has been carried out by the inventors of the instant invention to study the above reactions. The kinetics of these reactions were studied. These studies have shown that the key reactions involved silicon monoxide (SiO) and silicon carbide (SiC).
Silicon carbide can be produced via the reaction of silicon dioxide and a carbonaceous material according to reaction (1). The so-called Acheson reaction produces silicon carbide by heating a reaction mixture of silicon dioxide and carbon.
Khalafalla and Haas in Journal of the American Ceramic Society. 55:8(1972), pp. 414-417, describe a kinetic study of the carbothermal reduction of quartz under vacuum. The reaction studied was the reaction of an equimolar mixture of quartz and graphite to form silicon monoxide and not silicon or silicon carbide. The reaction studied by Khalafalla and Haas can be represented by the equation. EQU SiO.sub.2 +C=SiO+CO.
Khalafalla and Haas found that alkaline earth oxides such as calcium oxide. barium oxide. and magnesium oxide promote the above reaction under conditions of vacuum (low pressures). The reaction was studied under conditions of very low pressures. down to 10.sup.-6 torr, and a temperature of approximately 1400.degree. C. The reduction reaction was also studied with a deficiency of carbon relative to silicon dioxide. None of these forementioned conditions would suggest the preparation of silicon in a reduction furnace or the preparation of silicon carbide.
Henderson and Gutowski in Thermochimica Acta. 99(1986). pp. 309-316, disclose the effects of iron oxide. Fe.sub.2 O.sub.3, and chromium oxide, Cr.sub.2 O.sub.3, on the kinetics of the carbon-silica reactions in a glass-filled phenolic resin. Samples containing 10 to 25 weight percent oxide were tested. Henderson and Gutowski disclose that the oxides lowered the onset temperature at which the reaction of silica and carbon begin; however. Henderson and Gutowski go on to point out that the reaction rate for the silica-carbon reactions is not changed. and may in fact be decreased in the presence of the added metal oxides. Henderson and Gutowski do not suggest or demonstrate a smelting process in a silicon furnace to produce molten silicon or the preparation of silicon carbide as does the instant invention. Further. the very high proportions of iron oxide and chromium oxide disclosed by Henderson and Gutowski would result in the formation of ferrosilicon or chromium-silicon alloys and not the desired silicon of the instant invention.
Reaction (1). supra. is believed by the inventors to have a direct impact upon the rate of silicon generation and the utilization of raw materials and energy. The reaction of SiO.sub.2 and solid carbonaceous reducing agent (carbon) to form SiC is a critical step. In present submerged arc silicon furnaces, silicon dioxide and solid carbon are fed to the top of the furnace and molten silicon is tapped from the bottom of the furnace. The top one-third of the furnace operates at a temperature in the range of 1000.degree. to 1600.degree. C. The silicon dioxide and the solid carbon react to form SiC. Any carbon that remains unreacted enters a middle zone of the furnace and reacts with SiO to form SiC. according to Reaction (3) supra. The carbon in this case is competing with SiC for SiO which produces silicon. according to Reaction (5) supra. Maximum conversion of carbon to SiC in the upper zone of the furnace maximizes the rate of silicon formation and also silicon recovery from the raw materials fed. Unreacted carbon from the middle zone of the furnace entering the lower zone or hearth of the furnace could react with molten silicon to form SiC, according to the reaction EQU Si+C=SiC (7).
This reaction could also occur with silicon that may exist as a vapor in the lower zone of the furnace. The SiC that is formed, according to Reaction (7) supra. is found to be dense and non-reactive. Thus. the combination of unreactive SiC and the actual consumption of silicon formed results in lowered utilization of the starting raw materials.
The objective of the instant invention is to apply the experimental findings. supra. to improve the output from a silicon furnace via the carbothermic reduction of silicon dioxide. A second objective of the instant invention is to improve the utilization of raw materials and electrical energy in the manufacture of silicon. A further objective of the instant invention is to increase the production rate for a process for the preparation of silicon carbide.
It has been unexpectedly found that the addition of calcium compounds to Reaction (1). supra, significantly increases the rate at which SiO.sub.2 reacts with carbon to form SiC. Increases of reaction rate of as much as 200 percent for Reaction (1) were observed, as will be detailed in the examples infra. These findings can be applied directly to a process for the preparation of silicon carbide. These findings were then applied to a production-scale silicon furnace. It was found that the addition of a calcium compound as a constituent of the SiO.sub.2 or the solid carbonaceous reducing agent fed to a silicon furnace. or as a separate feed. increases the silicon production rate by as much as 12 percent. Further, raw material utilization was also improved significantly. Silicon recovery from the furnace was increased by as much as 10 to 15 percent. Additionally. the consumption of the graphite electrodes which normally account for approximately 10 percent of the carbonaceous reducing agents needed to reduce the silicon dioxide fed was reduced by as much as 30 to 40 percent. The observations of increased production rate and improved raw material usage are illustrated in the examples. infra. These improvements in production rate and raw material utilization are believed by the inventors to be the direct result of a significant increase in the rate of Reaction (1), supra, in which SiO.sub.2 and carbon are reacted to form SiC.